This R21 is submitted to pursue our investigations of the pathogenesis of Parkinson?s disease (PD) by exploring the role of extracellular alpha-synuclein (?-syn) in the neurodegenerative process of PD. While ?-syn is natively an intracellular protein, it can be recovered in the extracellular space due to, at least two, non- mutually exclusive mechanisms, namely atypical secretion and leakage from, healthy and damaged neurons, respectively. Once in the extracellular space, ?-syn is subject to oligomerization and/or modification (nitration or oxidization) that can trigger a microglial-derived inflammatory response, and, according to growing number of studies, a cell-to-cell transmission in both in vitro and in vivo models. We have compelling data showing that ?-syn is ingested by microglia via a receptor-mediated process, but the ?-syn species, which have been linked to PD (e.g. nitrated, oligomerized, and mutated) are less efficiently cleared than those which has not been linked to PD (e.g. native monomeric ?-syn). We thus hypothesize that the lesser the ingestion of extracellular ?-syn by microglia, the greater the microglial-derived inflammatory response and the cell- to-cell transmission and the ensuing neurodegeneration. The objective of this R21 is to characterize, in- depth, the relationship between ingestion, inflammation and cell-to-cell transmission triggered by extracellular ?-syn species both in vitro and in vivo. To achieve these stated goals, two specific aims (S.A.) are proposed. In S.A.1, in vitro investigations will: (i) complete the characterization of microglia receptor-mediated ingestion of selected ?-syn species; (ii) examine how different species of ?-syn elicit inflammation as measured by the production of selected inflammatory mediators; and (iii) assess the effects of Fc-mediated ingestion of the various forms of ?-syn and test subclasses of antibodies directed against selected forms of ?-syn to promote their microglial ingestion. How these strategies alter the ?-syn-related microglial production of inflammatory mediators will also be studied. All of these proposed experiments will be conducted in cultures of immortalized neonatal microglial cells using our novel assays to evaluate ?-syn ingestion and a panel of biochemical assays to quantify inflammatory markers in response to PD-linked and non-linked ?-syn species. In S.A.2, in vivo investigations will: (i) determine the effects of the deficiency in CD11b or SR-B2 ? two microglial receptors that we have identified as key determinants of ?-syn ingestion ? on brain inflammation and neuropathology in response to endogenous overexpression of wild-type or mutant A53T ?-syn; and (ii) examine the effects of the deficiency in CD11b or SR-B2 on brain inflammation, neuropathology, and cell-to-cell transmission in response to the injection of exogenous ?-syn. All of these proposed experiments will be conducted in transgenic mice expressing wild-type or mutant A53T ?-syn crossed with knockout CD11b or SR-B2 mice which will or will not be injected with ?-syn recombinant. This project is deemed significant and novel as it proposes an original pathogenic scenario, uses new methods, and may pave the way to new therapeutic avenues for PD.